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Thanks, everybody. That is interesting about the walstead method. My substrate consists of a 1" layer of cheap organic potting soil, covered with a thin layer of bentonite cat litter, covered with about 1 1/2" of gravel. Perhaps I have a walstead aquarium going without even realizing it. I wonder if I even need to add the flourish...it would be nice to get some cabomba going again.

Do not rejoice over me yet, o my enemy, for even if I fall, I will rise again. Even when I dwell in darkness, the Lord will be my light. Micah 7:8

I do not believe you can ever make a self sustaining natural aquarium unless you can provide water flow with out using any electrical devices. Even the walstad method requires filters.

But I assume that is because, in nature, you have both a comparatively huge water volume/exchange, and water circulation through plants and/or bottom substrate, which is the natural equivalent of biological filtration, right?

It's interesting that most of the creeks around here have little or no plant growth, even in the summer when days are >12 hours long, and even though we are at high altitude, so the sunlight is intense. I assume this is because of high gradient/fast water flow, low nutrient load, and (bingo!) extremely high aeration, thus very little CO2 in the water. But the rocks on the bottom are usually slippery, indicating a bacteria growth, which naturally filters the water.

Fascinating!

Do not rejoice over me yet, o my enemy, for even if I fall, I will rise again. Even when I dwell in darkness, the Lord will be my light. Micah 7:8